Amateur radio enthusiasts have been at the forefront of experimentation and development since the very earliest days of radio. All over the world, radio amateurs use a wide range of modes and frequencies to communicate. When surplus teleprinters became available in the early 1950s, radio amateurs adapted them for their own use and the first modern digital mode was born. Half a century later, RTTY is still widely used, and has been joined by a great number of other modes.
Most software for digital modes use a soundcard to communicate with the radio, and a simple circuit to switch the radio to transmit. Examples of the PTT circuits used may be found on Tom Sailer's soundmodem pages.
In order to transmit data over the air, you will need a suitable licence. In many countries you can get licence-exempt radios, but it may not be legal to use them for data modes. It is also possible to connect two PCs together via audio cables, and experiment with data modes.
For AX.25-based modes (packet radio, and APRS), Tom Sailer's soundmodem software will allow you to transfer data at up to 9600 baud, with a suitable radio. APRS uses 1200 baud data, which can be passed over the microphone and loudspeaker connections. High speed modes like G3RUH require specially-adapted radios, because the wide band data needs to have a flat audio response.
Soundmodem can be used as a KISS modem, which is treated as a serial device, or as an AX.25 network device which may be shared by several applications.
If you want to use soundmodem as an MKISS network device, you'll need to rebuild your kernel and install the mkiss kernel modules.
Run soundmodem as root:
If you have configured soundmodem as a KISS modem, you will need to set the device to be user-readable:
# chmod 666 /dev/soundmodem0
Xastir is an APRS application, which can send and receive short messages and positions.
If you set soundmodem up as a KISS modem, add it as a Serial KISS TNC in Interface -> Interface Control. If you set it up as an MKISS modem, add it as an AX25 TNC.
Most of the Linux Amateur Radio AX.25 HOWTO is relevant here. Install the libax25, ax25-apps and ax25-tools applications as appropriate from AUR.